Most petroleum hydrocarbon substances are derived from crude oil and, typically, comprise a mixture of short and medium length hydrocarbon compounds. The less volatile components of petroleum hydrocarbons can become environmental contaminants; they may remain in the environment for extended periods and become toxic to wildlife, flora and/or humans. Such contaminations are usually difficult, if not impossible, to observe unless gross contamination has occurred. Therefore, methods for the rapid and/or simple prediction of petroleum hydrocarbons in the environment (i.e. as may be found in soils, sludges and waterways) are desirable for monitoring environmental contamination and/or assessing a site.
For instance, with high urban growth, city fringes are gradually encroaching on areas that were formerly disused and/or predominantly industrial in nature. These sites present usable and valuable property for the further growth and development of many industrialised cities. However, many such sites have been contaminated by petroleum hydrocarbon leakage from their previous industrial uses, and some have been exposed to these materials time and time again. While these sites hold great potential, the environmental protection guidelines of most industrialised countries set safety standards for the minimum acceptable concentration of petroleum hydrocarbons in soils or other environmental matrices. Thus, to be placed in order for non-industrial reuse (e.g. residential or light-industrial uses), the amount of petroleum hydrocarbon on these sites must be reduced to acceptable levels for their intended future use. While many options are available for the treatment of contaminated sites, a significant portion of the time and costs involved in treating these sites is consumed in the monitoring of petroleum hydrocarbon over time. This is particularly true of the more sustainable remediation treatment techniques, as on site and/or real time techniques are not currently available for petroleum hydrocarbon monitoring.
In addition, causing an environmental contamination event can be considered as a serious offence with hefty penalties applicable to entities that flagrantly cause serious contamination. In these instances, regulators must rely on time-intensive and costly techniques to monitor the contamination event, with the delay in testing time, in turn, causing further delay in possible action to control the severity of the event.
Such testing situations typically involve the testing of numerous soil samples involving the extraction and quantitation of contaminant components, which is generally time consuming and labour intensive. For example, for the analysis of total petroleum hydrocarbons (TPHs) in soil, testing is usually carried out via supercritical fluid extraction of the TPH components from the soil samples followed by the quantitative analysis of the TPH either by gas chromatography-flame ionisation detection (GC-FID) and/or gas chromatography-mass spectrometry (GC-MS). GC-MS is particularly suitable for the quantitative analysis of the more volatile components of TPHs (i.e. compounds in the C6 to C10 carbon chain range), but often utilises a purge and trap method where a heated purge gas is used to introduce the TPH components to the GC column; a procedure that, alone, may take up to twenty minutes per sample. For the less volatile components of TPH (i.e. compounds in the C10 to C36 range), detection in soil or sediment samples can potentially be achieved by the direct extraction of the contaminants using a solvent such as methylene chloride, following sample sonication, and introduction of the TPH components to a GC column for GC-MS analysis or analysis by Fourier Transform (FT) IR reflectance (Sadler and Connell, 2003). However, these techniques, although they potentially provide accurate results, can also be time consuming and labour intensive. Moreover, none of these methods, which necessarily involve the use of sensitive equipment, are suited to on-site analysis of contaminants such as hydrocarbons.
Accordingly, a rapid and/or simple method for predicting the concentration of hydrocarbon contaminants, and particularly TPH components, in a site could provide significant cost advantages (i.e. in terms of reduced testing costs and/or the avoidance of delays in test results leading to productivity losses) and/or other advantages of rapid response, such as the capacity to undertake preventative measures to prevent further contamination and/or to limit contaminant spread.
Infrared (IR) spectroscopic techniques offer a possible alternative approach to supercritical fluid extraction and gas chromatography analyses of site contaminants. IR spectrometry distinguishes between chemical compounds by detecting the selective absorption of different IR wavelengths by vibrating chemical bonds; thus, each compound present in a sample being analysed that is IR active has a unique IR “spectral signature” enabling its identification and quantitation. However, while IR spectrometry-based techniques, such as diffuse reflectance infrared fourier transform (DRIFT) spectroscopy with partial least-squares (PLS) chemometrics (with mid-infrared (MIR) and near-infrared (NIR); Janik and Skjemstad, 1995; Reeves et al., 1999; and Cozzolino and Moron, 2003), have been employed in agricultural soil analysis for the detection of a large number of soil properties such as, for example, organic carbon, exchangeable cations, air-dry moisture and clay content, they have not to-date been used for the qualitative or quantitative analysis of contaminants such as petroleum hydrocarbon in complex mixtures such as soil, sediment, rock or mineral samples. The spectral peaks typically attributable to petroleum hydrocarbon contaminants may also occur in frequency regions due to the presence of naturally-occurring organic matter (NOM) and/or be masked by other factors. For example, using DRIFT, where the radiation penetrates a short distance (a few tens of micrometers) into soil, quartz (as sand) and clays can give particularly strong MIR spectral signatures which can, as a result, overlap with peaks potentially useful for TPH determination.
The present invention is directed at IR spectrometry-based methods for predicting hydrocarbon contaminants in an environmental sample, particularly a soil, sediment, rock, mineral or other solid sample, which may overcome one or more of the problems associated with the prior art.